Flood disaster management implies not letting the excess runoff water flow suddenly and intensively in the drainage network. After the unprecedented floods of 1954, flood management works were taken up in a planned manner by the State Governments.

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The main thrust of managing floods in different river basins was to modify the floods through specific structural measures such as reservoirs, embankments, channel improvement, town protection and river training works. The various measures adopted for flood mitigation may be categorised into two groups viz., structural and non-structural.

The main steps for flood disaster management are briefly discussed below:

(1) Flood Forecasting:

Flood forecasting involves giving prior information regarding the occurrence of floods. This is essential and is extremely useful for taking timely action to prevent loss of human lives, livestock and movable property.

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The Central Water Commission (CWC) started flood forecasting in November 1958 when the first forecasting station was established at Old Railway Bridge, Delhi. Since then it has been extended to cover almost all the major inter-state flood prone rivers of the country. At present there are 166 flood forecasting stations on various rivers in the country which includes 134 level forecasting and 32 inflow forecasting stations. River-wise break-up is given in Table 8.13.

The Flood Forecasting Network covers the 14 States and one Union Territory in addition to NCT of Delhi. State-wise numbers of flood forecasting centres are given in Table 8.16.

These centres issue daily-flood forecasts and warnings throughout the flood season from May to October. For achieving greater accuracy, the Central Water Commission recently established a procedure of carrying out a self-analysis and appraisal of the forecasting network at the end of the monsoon season.

The Flood Forecasting involves the following four main activities:

(i) Observation and collection of hydrological and hydro-meteorological data;

(ii) Transmission of Data to Forecasting Centres;

(iii) Analysis of data and formulation of forecast; and

(iv) Dissemination of forecast.

Table 8.13 Flood Forecasting Stations in River Systems:

SI. No.

Name of River Systems

No. of Flood Forecasting Stations

Level

Inflow

Total

1.

Ganga & Tributaries

71

14

85

2.

Brahmaputra & Tributaries

27

—

27

3.

Barak-System

2

—

2

4.

Eastern-Rivers

8

1

9

5.

Mahanadi

2

1

3

6.

Godavari

13

4

17

7.

Krishna

2

6

8

8.

West Flow Rivers

9

6

15

Total

134

32

166

Table 8.14 India: Flood Forecasting Centres:

SI. No.

State/Union territory

No. of Flood Forecasting Stations

Level

Inflow

Total

1.

Andhra Pradesh

8

07

15

2.

Assam

23

—

23

3.

Bihar

32

—

32

4.

Chhattisgarh

01

—

01

5.

Gujarat

06

04

10

6.

Haryana

—

01

01

7.

Jharkhand

—

04

04

8.

Karnataka

01

03

04

9.

Madhya Pradesh

02

—

02

10.

Maharashtra

05

02

07

11.

Orissa

10

01

11

12.

Uttaranchal

01

02

03

13.

Uttar Pradesh

31

04

35

14.

West Bengal

11

03

14

15.

Dadra & Nagar Haveli

01

01

02

16.

NCT of Delhi

02

—

02

All India Total

134

32

166

On an average, 6,000 forecasts at various places in the country are issued during the monsoon season every year. The analysis of the forecasts issued during the last 25 years (1978 to 2002) indicates that accuracy of forecasts has consistently increased from around 81% to 98%. Forecast is considered accurate if forecast water level is within ±15 cm of actual water level of the inflow forecast (i.e., discharge) is within ± 20% of actual discharge.

A Memorandum of Understanding was signed by India and Nepal in 1988 according to which 45 Hydrological and Hydro-meteorological Stations were to be set up in India and Nepal for issuing flood forecasts to benefit both the countries.

Bulletins are also updated on CWC Web site www.cwc.nic.in for wider publicity among user agencies during flood season.

Forecast Dissemination:

The utility of flood forecastes is dependent on both accuracy and timeliness. The organisations responsible for flood-protection, warning and flood-fighting works should be informed about the incoming flood as early as possible so that the required action is planned and activities set into operation with least possible time delay.

A “Flood Forecast” received too late to take the necessary flood fighting measures is of “No” use. It is, therefore, imperative to take minimum time in dissemination of forecast.

Forecast Bulletins:

Flood forecasts and warning which are formulated by various flood forecasting centres are supplied in the form of “DAILY WATER LEVEL AND FLOOD FORECAST BULLETINS” to concerned Civil and Engineering Authorities on Wireless/Telephone/by Special messenger/Priority Telegrams, depending upon the urgency and available mode of communication media.

Control Rooms:

Generally, the State Governments set up “Central Control Rooms” at State and District Headquarters which receive these forecasts and disseminate the warning to the affected areas and organise relief as well as rescue operations. The forecasting centres also send the forecasts to the “ALL INDIA RADIO” stations, “DOORDARSHAN” and the local “NEWSPAPER” for wider publicity.

On receipt of “Fresh Information” a revised forecast is issued, if the situation warrants. During high flood stages the “Control Room” of the forecasting centre works round the clock and keeps informed the flood fighting agencies about the latest river position. They work in close collaboration.

2. Reduction of Runoff:

Reduction of runoff is one of the very effective methods of flood disaster management. Runoff can be reduced by inducing and increasing infiltration of the surface water into the ground in the catchment area. This can be done by large scale afforestation particularly in the catchment area. Afforestation helps in reduction of runoff in the following ways:

(i) The canopy of the forest cover intercepts the falling raindrops and the roots, the leaf litter and humus are capable of holding water.

(iv) Reduction in stream sediment load reduces siltation and helps in maintaining the water accommodating capacity of the rivers.

In the Indo-Gangetic plain, the runoff can be reduced by artificially inducing infiltration by digging wells along the beds of ephermal channels. A series of dug wells helps in storing and channelising the surface water. There is vast scope of using this method in the Northern Plain of India due to soft soil and vastness of aquifers.

The flood peaks can be reduced by construction of dams and detention basins. Dams have the capacity of holding huge quantity of water during the flood period and help in reducing flood peak volume of water.

Water stored in reservoirs created by constructing dams can be allowed to flow down the stream under controlled conditions depending upon the accommodating capacity of the river downstream the dam. A number of reservoir projects have been completed in India since the launching of the National Flood Control Programme in 1954.

These dams have helped in mitigating flood-peak in the downstream reaches. Notable among these are Tilaiya, Konar, Maithon and Panchet Hill dams in the Damodar Valley System, Bhakra Dam on the Satluj, Hirakud Dam on the Mahanadi, Pong Dam on the Beas, Nagarjun Sagar and Tunghbhadra on the Krishna and Ukai Dam on the Tapi. All these dams have afforded reasonable degree of protection to about 13.64 lakh hectares of land.

Apart from dams as described above ponds, tanks and surface storage structures also check flood and help in harvesting water for dry seasons. Other type of detention basins includes natural depressions such as marshes in plains and old quarries and mines.

4. Reducing Flood Levels:

Flood levels can be reducing in the following ways.

(i) Stream Channelisation:

A close network of canals reduces flood hazard to a great extent because flood water flowing in the river can be diverted to canals. Canals serve as temporary storage and hold water as its flood waves move downstream. Thus they help in reducing the severity of the flood.

(ii) Channel Improvement:

Channel improvement is done by deepening, widening, straightening, lining and cleaning out of vegetation and debris from the river channel. These changes in the river channel increase the flood conveyance capacity of the river. Channel improvement is supplemented by bank stabilisation by constructing ripraps, dykes or spurs and planting deep root trees on embankments. In a meandering river, meander loops impede drainage and retard disposal of flood water. Whenever, the river meanders become extremely sharp, they can be straightened by artificially cutting individual or a series of bends. This method can be applied to the meandering courses of the rivers like the Gandak, the Gomati, the Rapti, the Kosi, etc.

(iii) Flood Diversion:

Flood diversion is the process of diverting the flood water in marshes, lakes, the depressions and spreading it thinly over paddy fields and desert drylands. One such scheme in the Ghaggar Riversion Scheme which diverts 340 cumecs (cubic metres per second) of water before its entry into Rajasthan into the depressions and the areas between the sand dunes. In this way, discharge of water in the Ghaggar River is kept within the safe limits during the flood period.

5. Protection against Inundation (Construction of Embankments):

Building of embankments was considered to be the only way of controlling floods in 1940s. It is still considered to be one of the very effective devices against inundation of the inhabited areas and agricultural land. Construction of embankments has been taken up at a large scale in India. Between 1954 and 1978, 10,821 km long embankments had been built. By March 2000, more than 33,630 km of new embankments had been constructed.

Most of the embankments have been constructed in north India where the Brahmaputra valley of Assam, northern parts of Bihar, Uttar Pradesh (Ganga, Yamuna and Ghaghara) and Punjab (Satluj, Beas and Ravi) are the main beneficiaries. In south India, embankments have been constructed mainly in the deltaic parts of the Mahanadi, the Godavari, the Krishna and along the banks of Penneru (Fig. 8.16).

As mentioned earlier, the Brahmaputra valley in Assam is the most frequently and severely flooded part of India. As such it is also the most heavily embanked part of the country. About one third of the total embankments of India have been constructed in Assam alone.

The largest construction of embankments has taken place along the Brahmaputra River itself. Efforts have been made to control floods in the Brahmaputra river by constructing embankment at several places. At present the total length of embankments along the Brahmaputra is 934 km and 2,400 km on its various tributaries. These embankments provide protection to an area of 13.27 lakh hectares out of a total flood-prone area of 30 lakh hectares.

Next to Assam, Bihar is the most heavily embanked state, About 20 per cent of the total embankments of India have been constructed in Bihar. With the increase in flood prone area over the years from 2.5 million hectares in 1952 to 6.89 million hectares in 1994, the length of embankments has also grown from 160 km in 1952 to 3,465 km in 1998 i.e., an increase of almost 22 times.

The Kosi, and the Burhi Gandak are the most heavily embanked rivers. The Gandak, the Bhagmati. the Son and the Mahananda also have long stretches of embankments. These embankments have provided considerable protection to large area particularly in north Bihar.

However, there are severe limitations of embankments as a flood control device. As a matter of fact, they are not so much a method of flood control as flood transfer. Embankments may protect the neighbouring areas but they often cause floods in the downstream areas.

In the event of high flood levels, the embankments may develop breaches and floods cause heavy damage to life and property in the low lying areas near the embankments. Construction of embankments put limits to the river channel.

The sediment which was to be deposited in a much wider area in the absence of embankments is deposited in a limited river channel. Thus the river bed rises at an accelerated rate and consequently the flood water level rises. Under such circumstances the flood water may overflow the embankments or there may be breaches in the embankments due to intense hydraulic pressure.

Under both the circumstances, the flood situation takes a serious turn and causes untold miseries to the people living in the neighbouring areas. Thus whereas constructing embankments is a very useful method of flood control it can lead to a very serious flood situation.

The Rashtriya Barh Ayog report states. “Embankments are not a feasible measure of flood protection in cases where the country runoff draining into the river is so large as to inundate appreciatively the area protected by the embankments from river spills, during periods when the river is running at high flood stages.”

6. Flood Plain Zoning (FPZ):

Flood plain zoning is another very effective method of flood management. It is based on information regarding flood plains, particularly the identification of floodways in relation to land use. Detailed maps of flood prone areas are prepared after a thorough study of flood cycles.

Some areas are more prone to floods than the others. Different zones are identified and demarcated. After that necessary control is exercised with respect to land use. In Figure 8.17 area ‘A’ is the main channel of the river. It is the flood way which is totally prohibited zone. No construction is allowed in this zone. Area ‘BB’ is the regulatory flowing fringe and marks the extent inundation design flood. This is called restrictive zone. Beyond that is the secondary hazard zone marked by ‘CC’. This is the extent of largest flood expected.

Legislative measures are adopted to restrict engineering, industrial and economic activities in these zones. Under the legislative obligations, construction of buildings or unwanted industrial units, etc. is not allowed. The proper use of flood plains is shown in Figure 8.18.

Recognising the significance of FPZ as an effective device of flood control, the Central Flood Control Board mooted the idea in 1957 to demarcate flood zones to prevent indiscriminate settlement in flood plains. The increasing trend in flood damages was observed in India even during the 1970s and the State Governments were requested to give due attention to development of flood plains in a regulated manner. A model bill on Flood Plain Zoning was circulated to State Governments as early as 1975 with a request to enact suitable legislation for restricting the encroachment of flood plains and for its development in a regulated manner.

The main features of the model bill were:

(a) Flood zoning authority,

(b) Delineation of flood plain,

(c) Notification of limits of flood plains,

(d) Restrictions on use of flood plains,

(e) Compensation, and

(f) Power to remove constructions after prohibition.

However, the response from the State Governments, except Manipur, is not encouraging. The State Governments are being continually requested to give serious consideration to zoning of flood plains and their development in a regulated manner through administrative measures pending enactment of suitable legislation.